Recent developments in the advanced information society are remarkable, and further rapid developments can be found in “mobile devices”. The great demand for the “mobile devices” is regarded as a possible main factor in future semiconductor industries. However, in addition to the existing demands for high-speed operations, low power consumption, and high capacities of semiconductor integrated circuits, there are new demands for non-volatility of information. In such a trend, attention has been drawn toward a novel memory device that has been developed by combining an excellent ferromagnetic storage technique and semiconductor integration electronics for non-volatile high-density recording (disclosed in Non-Patent Document 1, for example). This device is called a magneto-resistive random access memory (hereinafter referred to as “MRAM”), and a magnetic tunnel junction (hereinafter referred to as “MTJ”) having a thin insulating tunnel barrier interposed between ferromagnetic electrodes is used as a memory element.
A MTJ has a tunneling magneto-resistance (hereinafter referred to as “TMR”) effect with which the tunnel resistance varies with the relative magnetization direction between the ferromagnetic electrodes. Accordingly, using the MTR effect, the magnetization configuration of a ferromagnetic member can be electrically detected. With a MTJ, the non-volatile information using ferromagnetic members can be positively applied to the semiconductor integration electronics.
Referring now to FIG. 8, an example of the prior art is described. As shown in FIG. 8, an MRAM memory cell 100 has a 1-bit memory cell formed with a MTJ 101 and a MOS transistor 103. The MTJ 101 is a tunnel junction that includes a first ferromagnetic electrode 105, a second ferromagnetic electrode 107, and a tunnel barrier 108 formed with an insulator interposed between the two electrodes.
The source (S) of the MOS transistor 103 is grounded (GND), and the drain (D) is connected to the ferromagnetic electrode 107 of the MTJ 101 with a plug PL. The ferromagnetic electrode 105 of the MTJ 101 is connected to a bit line BL. A rewrite word line 111 is disposed to cross the bit line at right angles, while being electrically insulated from the MTJ 101 and the other lines by an insulating film 115 immediately above or below the MTJ 101. A read word line WL is connected to the gate electrode G of the MOS transistor 103.
Since the magnetization directions can be maintained in a non-volatile manner in a ferromagnetic member, a MTJ can store binary information in a non-volatile manner by switching the relative magnetization configuration of the ferromagnetic electrodes to parallel magnetization or antiparallel magnetization. Also, in a MTJ, the tunnel resistance varies in accordance with the relative magnetization configuration between the two ferromagnetic electrodes due to the TMR effect. Accordingly, utilizing the tunnel resistance in accordance with the magnetization configuration that switches between parallel magnetization and antiparallel magnetization, the magnetization configuration of the MTJ can be electrically detected.
Information rewrite is performed by making the coercive forces of the two ferromagnetic electrodes 105 and 107 different from each other in the MTJ 101, or inverting the magnetization of the ferromagnetic electrode with the smaller coercive force or with an unfixed magnetization direction while fixing the magnetization direction of the other ferromagnetic electrode. Hereinafter, a ferromagnetic member having a magnetization direction to be inverted will be referred to as a free layer, and a ferromagnetic member having a magnetization direction not to be inverted will be referred to as a pin layer. More specifically, currents are applied to the bit line BL and the rewrite word line 111 that cross each other on a selected memory cell. Using the synthetic magnetic field of the magnetic fields induced by the currents, only the magnetization configuration of the MTJ 101 in the selected memory cell 100 is switched to parallel magnetization or antiparallel magnetization. Here, the values of the currents to be applied to the lines are set so that the magnetization of the MTJ 101 of each unselected memory cell connected to the bit line BL and the rewrite word line 111 to which the selected memory cell is connected is not inverted only through one of the bit line BL and the rewrite word line 111.
Information is read out by energizing the MOS transistor 103 through application of a voltage to the read word line WL connected to the selected cell, and then applying a read driving current to the MTJ 101 via the bit line BL. Since the tunnel resistance varies in accordance with the magnetization configuration that can be switched between parallel magnetization and antiparallel magnetization due to the TMR effect in the MTJ 101, the magnetization configuration of the MTJ 101 can be determined by detecting a voltage decrease (hereinafter referred to as “output voltage”) due to the read driving current in the MTJ 101. The following is the documents relating to the above described techniques.
1) K. Inomata, “Present and Future of Magnetic RAM Technology”, IEICE Trans. Electron., Vol. E84-C, 2001, pp. 740-746.
2) H. Ohno, D. Chiba, F. Matsukura, T. Omiya, E. Abe, T. Dietl, Y. Ohno, and K. Otani, “Electric-Field Control of Ferromagnetism”, Nature, Vol. 408, 2000, pp. 944-946. (also described later)
3) D. Chiba, M. Yamanouchi, F. Matsukura, and H. Ohno, “Electrical Manipulation of Magnetization Reversal in a Ferromagnetic Semiconductor”, Science, Vol. 301, 2003, pp. 943-945. (also described later)